1. Field of the Invention
This invention relates to an image input device such as an electronic camera using accumulation, and more particularly, to an exposure control circuit of an image input device that speeds up the photographing of a subject having a wide dynamic range.
2. Description of the Related Art
Recently, various solid-state imaging elements including CCD image sensors and MOS image sensors have been developed. Those solid-state imaging elements have found application to a variety of image input devices such as electronic cameras.
While some subjects often have a dynamic range as wide as about 80 dB, solid-state imaging elements generally have an imaging capability (a dynamic range) of up to about 50 dB. Therefore, with such solid-state imaging elements, it is impossible to photograph a subject with a wider dynamic range than 50 dB. For ranges wider than 50 dB, the output of the imaging element can be saturated in the high luminance region, causing what is called white compression, or be short of exposure in the low luminance region, creating a blacked-out portion in the image.
To overcome those disadvantages, Japanese Patent Application No. 1-334508 discloses a device that broadens the dynamic range by accumulating the image signal from the solid-state imaging element. FIG. 1 is a schematic diagram for an important part of the disclosed device.
In the FIG. 1, the image signal read at a high speed from the solid-state imaging element (AMI) 2, which electrically images the subject (not shown), is amplified to a specified signal level at the preamplifier 4. The amplified signal undergoes various processes including clipping at the signal processing circuit 6 and is digitized at the A/D converter 8. The digitized image signal is supplied via the adder 10 to the frame memory 12, which, together with the adder 10, carries out accumulation and stores the resulting signal in frames sequentially.
The video processor 14 extracts the luminance signal component Y from the accumulated image signal having an extended dynamic range and at the same time, divides the rest of the signal into the three primary color components R, G, and B. The luminance signal component Y is log-converted into log Y at the log amplifier 16a of the dynamic range control circuit 16. The resulting signal is supplied via the two-dimensional filter 16b, which eliminates luminance irregularities, to the dynamic range gain controller (DGC) 16c. This controller 16c adjusts the dynamic range of the luminance signal component Y to that of the image monitor and supplies the output to the adder 16e, which also receives the output from the log amplifier via the delay circuit 16d. The adder 16e calculates the compression coefficient of the dynamic range.
On the other hand, the three primary color components R, G, and B are log-converted into log R, log G, and log B at the log amplifiers 18r, 18g, and 18b, respectively. These resulting signals are supplied via the delay circuits 20r, 20g, and 20b to the adders 22r, 22g, and 22b, respectively. These adders 22r, 22g, and 22b add the compression coefficient to the inputs. The resulting signals undergo reverse conversion at the reverse log amplifiers 24r, 24g, and 24b and then are supplied to D/A converters 26r, 26g, and 26b, which produce the signal components whose dynamic range is compressed.
Making use of the fact that accumulation of the signal reduces random noises, the device thus constructed provides a wide dynamic range.
For such a device, however, there is no detailed description of the relationship between the number of accumulations by the solid-state element and the exposure time. If the number of accumulations is n, the signal value will be n times larger and the noise value n.sup.1/2 times larger, so that the imaging dynamic range can be increased up to n.sup.1/2 times. That is: i) the more the number of accumulations increases, the wider the dynamic range, but the longer the time required for photographing becomes; ii) when the subject to be photographed has a narrow dynamic range, unnecessary accumulations prolongs photographing time wastefully; and iii) when the subject with a high luminance is photographed, it is necessary to shorten the exposure time of the solid-state imaging element, whereas when the subject with a low luminance is photographed, it is necessary to lengthen the exposure time. Therefore, the number of accumulations and the exposure time had to be set to suitable values depending on the dynamic range and luminance of the subject to be photographed.
Published Unexamined Japanese Patent Application No. 63-201406 discloses a device for controlling exposure time. This device, using a nondestructive sensor, reads image data at a high speed and stores more than one image with different exposure times during one exposure period.
With such a device, however, while the exposure time is controlled, the exposure is not controlled to a suitable amount by a diaphragm as with the prior art described above. Namely, the exposure time is controlled depending on the state of the subject, but for a dark subject, the exposure time tends to be longer than necessary.